Hydroxybenzoic acids are useful as intermediates in the manufacture of many valuable materials including pharmaceuticals and compounds active in crop protection, and are also useful as monomers in the production of polymers. In particular, 2,5-dihydroxyterephthalic acid (Formula I, “DHTA”) is a useful monomer for the synthesis of high strength fibers such as those made from poly[(1,4-dihydrodiimidazo[4,5-b:4′,5′-e]pyridine-2,6-diyl)(2,5-dihydroxy-1,4-phenylene)].

Various preparations of 2,5-dihydroxyterephthalic acid and other hydroxybenzoic acids are known. Marzin, in Journal fuer Praktische Chemie, 1933, 138, 103-106, teaches the synthesis of 2,5-dihydroxyterephthalic acid from 2,5-dibromoterephthalic acid (Formula II, “DBTA”) in the presence of copper powder.

Singh et al, in Jour. Indian Chem. Soc., Vol. 34, No. 4, pages 321-323 (1957), report the preparation of a product that includes DHTA by the condensation of DBTA with phenol in the presence of KOH and copper powder.
Rusonik et al, Dalton Transactions, 2003, 2024-2028, describe the transformation of 2-bromobenzoic acid into salicylic acid, benzoic acid, and diphenoic acid in a reaction catalyzed by Cu(I) in the presence of various ligands. A tertiary tetraamine minimizes the formation of diphenoic acid in use with Cu(I).
Comdom et al, Synthetic Communications, 32(13), 2055-59 (2002), describe a process for the synthesis of salicylic acids from 2-chlorobenzoic acids. Stoichiometric amounts of pyridine (0.5 to 2.0 moles per mole of 2-chlorobenzoic acid) are used such as at least 1.0 mole pyridine per mole 2-chlorobenzoic acid. Cu powder is used as a catalyst along with the pyridine.
The various prior art processes for making hydroxybenzoic acids are characterized by long reaction times, limited conversion resulting in significant productivity loss, or the need to run under pressure and/or at higher temperatures (typically 140 to 25° C.) to get reasonable rates and productivity. A need therefore remains for a process by which 2,5-dihydroxy terephthalic acid can be produced economically; with low inherent operational difficulty; and with high yields and high productivity in both small- and large-scale operation, and in batch and continuous operation.